PhotSat detector tests at Montsec Observatory facilities

Authors: J.M. Carrasco, G. Ferrer, E. Masana, J. Maller

Date: 12 March 2025

The detectors intended for the PhotSat space mission have been successfully delivered to the Institute of Space Studies of Catalonia (IEEC). These devices are now undergoing a comprehensive test campaign to evaluate their performance under a variety of conditions, replicating those expected during the mission. The characterization process includes laboratory tests in a vacuum environment, at different controlled temperatures, under diverse illumination settings, and in radiation-exposed scenarios, among others; and on-sky tests. These tests are essential for ensuring that the detectors can withstand the operational demands of space and deliver high-quality scientific data.

Fig. 1: The detector (left) and optical device (right) used for the tests done at Montsec Observatory facilities during March 2025 (PhotSat consortium)

As part of this validation campaign, a dedicated team from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) carried out a set of on-sky tests in March 2025 at the Montsec Astronomical Observatory (OAdM). These tests aimed to emulate the image acquisition and processing conditions expected for PhotSat. To this end, the team captured night-sky images using one of the detectors mounted on a telephoto lens designed to replicate the satellite’s field of view. The exposures were set to get the expected signal during the actual PhotSat observations, in both visible and UV range.

Fig. 2: The ICCUB team in charge performing the tests at Montsec Observatory with the PhotSat detector (PhotSat consortium).

The goals of these tests are multifaceted. One key goal is to simulate the on-board image stacking process that will be used in orbit. During operations, PhotSat will combine multiple exposures of the same field into a single co-added image, which will then be downlinked to Earth. By reproducing this process with ground-based observations, the team can assess the response of the detector across successive frames and evaluate potential effects such as fixed-pattern noise, thermal drift, and pixel response non-uniformity.

In addition, the dataset collected serves as a testbed for the PhotSat calibration and image processing pipeline. The analysis of the raw and calibrated data helps refine the algorithms for bias correction, flat-fielding, cosmic ray removal, and photometric calibration that will be deployed both on-board and in the ground-segment.

Fig. 3: Coadded images of M42 nebula before (left) and after (right) calibration, observed with EHD detector at Montsec Observatory.

Fig. 4: Calibrated coadded images for M42 (left, detail) and for the Pleyades field (right).

The first results include co-added and calibrated images of well-known astronomical fields, such as the Orion Nebula (M42) and the Pleiades cluster (M45), which have been used to assess both the scientific performance and the calibration stability of the system. These benchmarks allow for a quantitative comparison between the expected and measured photometric response of the detector.

This effort represents a critical milestone in the preparation for the PhotSat mission. The lessons learned during these ground-based tests will directly inform the calibration strategy and help ensure the reliability and scientific return of the mission once in orbit.